I've never seen OO buckshot used in 3gun.. Usually birdshot #7 1/2 or even #8.

Some guys run reduced recoil slugs... I'm a thin guy w/o a lot of 'padding' and I just use a recoil-friendly technique instead of reduced power loads. Simple as pushing hard forward with the forward hand, pulling back hard with the grip hand, and only light/average shoulder pressure = no bruised shoulders after lots of slugs. Guys who pull back hard in to the shoulder get hammered when shooting full power 1oz slugs. Poor suckers.

I shoot an m2 in 12 and my wife shoots an m2 in 20.... I'd be lying if I didn’t say that her 20 is much easier to manage, shoot "quick" and shoot on the move than my 12. She has never been at a disadvantage with it, even at extended ranges.

If you are smaller stature or very recoil sensitive don’t let all the "bigger is better" boys steer you away from trying a 20 gauge.

as for ammo for shotgun in 3 gun 99% of what you will shoot will be shot with an occasional slug are two thrown in.